1. Field of the Invention
The invention relates generally to current collectors, anodes, and rechargeable batteries. More particularly, the invention provides a current collector for use in electrochemical devices, and an anode and a rechargeable battery each including the current collector.
2. Description of the Related Art
With the recent trend of widespread use of portable electric devices such as video camera, digital still camera, cellular phone, notebook computer, and so forth, the miniaturization, weight reduction, and extended life, of these devices have been eagerly desired. In order to meet the demands for electronics applications, the development of batteries, and of rechargeable batteries, in particular, has been actively pursued as to satisfactory power sources that are feasible of achieving the miniaturization, lighter-weight, and higher energy density.
Among others, rechargeable lithium-ion batteries, which utilize the insertion and extraction of lithium ions during charge-discharge reaction, have been met with a high degree of expectation. This is for the reason of its energy density higher than the density obtained with lead battery or nickel-cadmium battery.
The rechargeable lithium-ion battery is formed by including a cathode and an anode, and also an electrolyte. The anode is provided with an active anode material layer, and this active anode material layer is formed to include active anode material which is capable of participating in the charge-discharge reaction.
As the active anode material, carbon containing materials have been widely used. However, since further improvement in battery capacity is desired recently in response to the requirements for higher performance and expanded functionality of portable electric devices, silicon materials have been investigated. This is because the theoretical capacity (4199 mAh/g) of silicon is markedly higher than that of graphite (372 mAh/g), and a large improvement in the battery capacity can therefore be expected from silicon. In this regard, other than the single silicon element, several compounds and alloys of silicon have also been investigated.
When silicon is used as the active anode material, however, the layer of active anode material either expands or contracts severely, resulting in the deformation of the current collector, and possibly in the damage or separation of the active anode material layer. As a result, it becomes for the cycle characteristic, as one of the important properties of the rechargeable battery, to be deteriorated with relative ease.
Incidentally, numerous investigations on rechargeable lithium-ion batteries have been carried out to improve various performances. To be more specific, for example, in order to realize a battery system of sealed-type being formed with low magnetic susceptibility materials, an anode current collector is used by having a screen-shape formed by chemical etching process (see, Japanese Unexamined Patent Application Publication No. 2000-100475, for example). As another example, in order to improve charge-discharge cycle characteristics by increasing the adhesion of a current collector and a thin active material layer, a metal foil, which is anticorrosion processed with a chromium containing layer, is employed as an anode current collector (see, Japanese Unexamined Patent Application Publication No. 2002-319407, for example). In this case, after copper grain powders are plated, a copper foil is also employed as an anode current collector, which is provided thereon with a dense copper layer formed by plating process so as not to damage an irregular structure formed previously with the grain powders (see, Japanese Unexamined Patent Application Publication No. 2002-319408, for example). As still another example, in order to improve cycle characteristics without increasing the thickness of anode current collector, an anode current collector, which is provided thereon with protrusions, is used (see, Japanese Unexamined Patent Application Publications No. 2004-207112, 2007-103197, and 2008-41307, for example).